Field of the Invention
The present invention relates to a technique for enhancing in-plane uniformity of a surface light source device.
Description of the Background Art
A liquid crystal display element provided in a liquid crystal display device does not emit light by itself. For this reason, the liquid crystal display device includes a backlight device (a surface light source device) as a light source for illuminating the liquid crystal display element at a back surface side of the liquid crystal display element.
Moreover, the liquid crystal display element includes a color filter and transmits, through a color filter, only light having a part of wavelengths in light emitted from a fluorescent lamp for emitting white light in a continuous spectrum, thereby extracting display colors of red, green and blue to perform color representation. In the case in which only light in a part of wavelength bands is cut out of light source light in the continuous spectrum to obtain the display color, thus, a transmission wavelength band of the color filter provided in the liquid crystal display element should be set to be narrower if color purity of the display color is to be increased in order to enlarge a color reproduction range. For this reason, if the color purity of the display color is to be increased, there is a problem in that a quantity of light transmitted through the color filter is decreased, resulting in reduction in a luminance.
A fluorescent lamp to be generally employed as a light source of a backlight device has a light emission spectrum with a peak in a wavelength shifted to an orange color of approximately 615 nm in a red wavelength range based on a characteristic of a phosphor. If the color purity is to be increased in a wavelength region of 630 to 640 nm which is preferable as pure red in the red color, therefore, there is a problem in that the quantity of transmitted light is extremely reduced, resulting in significant reduction in a luminance. In order to enlarge the color reproduction range while minimizing a light loss caused by the color filter, it is necessary to employ a light source for emitting light having a narrow wavelength bandwidth. In other words, it is necessary to employ a light source for emitting light having high color purity.
As a measure for solving the problems, in recent years, there has been proposed a liquid crystal display device including, as a light source, a monochromatic LED or laser having a narrow wavelength width, that is, high color purity. In particular, the laser has excellent monochromaticity and high light emission efficiency. Therefore, it is possible to obtain a liquid crystal display device having a large color reproduction area, providing an image of high quality with a high luminance, and having low power consumption.
In recent years, the light emission efficiency of the LED has also been enhanced. For this reason, a backlight device for a liquid crystal display element or the like has also used a technique for using three types of LEDs that respectively emit light having red (R), green (G), and blue (B) which are three primary colors of light and mixing colors of RGB light emitted from the three types of LEDs, thereby obtaining white light.
In order to enhance utilization efficiency of light emitted from a light source, furthermore, there has been used a directly under type backlight device for directly supplying light from a back surface to a liquid crystal display element.
In the liquid crystal display device including the backlight device, it is desirable to implement a surface light source device of high quality which has small luminance unevenness and color irregularity with a simple structure.
In this regard, for example, Japanese Patent Application Laid-Open No. 2006-286906 discloses the technology for attaching a light emitting diode device for adjusting light traveling along a central axis of a lens to the lens covering a light emitting diode chip or a periphery thereof, thereby improving luminance unevenness and color irregularity in a liquid crystal display device.
The light emitting diode device described in the Japanese Patent Application Laid-Open No. 2006-286906 is designed for a divergence angle of an LED. By adjusting a direction of light travelling along a central axis of a lens, that is, a direction of light travelling directly toward a display surface, it is possible to obtain white light without unevenness by mixture of LED light having red, green and blue colors.
In the case in which plural types of light sources having greatly different divergence angles are used for a planar light source of a display device, however, a way for travel of light of each light source emitted out of the light emitting diode device is varied due to a difference in light distribution. For this reason, a difference is made in the distribution of light illuminating a display surface for every light source so that luminance unevenness is caused. In particular, a distribution difference on the display surface for different colors causes color irregularity and is thus recognized visually.
As an example, a laser beam has a narrow wavelength bandwidth. By using a laser for the light source, therefore, it is possible to enhance color reproductivity. However, the laser beam has a very small divergence angle. Even if the same optical system as that for LED light is applied, therefore, it is hard to have equivalent expansion to the LED light. When distribution of a laser beam is different from that of the LED light over the display surface, luminance unevenness and color irregularity are caused.
In the case in which the plural types of light sources having different divergence angles are simultaneously used for a surface light source device, thus, it is necessary to match a way for spread of each light source light. Even if the same optical system is applied to each light source light, therefore, it is hard to create a uniform surface light source. If two types of light sources are caused to have structures for creating surface light sources respectively, moreover, the structures are made complicated so that a manufacturing cost is increased.
Also in the case in which a light source of a single type is used for the surface light source device, moreover, it is necessary to enhance in-plane uniformity of the light source light in order to display an image of high quality.
In consideration of the problems, therefore, it is an object of the present invention to display an image of high quality in a display device using a surface light source device having a simple structure.